Error: object 'r' not found

[handaanmol]

Whenever I run the plumberize.R file, it shows me the following error
Loading required package: httr
Error in stopOnLine(lineNum, line, "Multiple @serializers specified for one function.") :
Error on line #95: '#* @serializer custom_json' - Multiple @serializers specified for one function.

r$run(port=8000)
Error: object 'r' not found

Can you please help me out? I am missing something here?